Aquaculture diminishes native fish species in Fiji

The invasive fish species of Tilapia and Mosquitofish coming from badly constructed fish farms are diminishing native fish species in Fiji. This is the result of a six-year study to 20 catchments on the Pacific islands. ‘Invasive Alien Species’ is today’s International Day for Biological Diversity theme.

The research was done by Wetlands International scientists and Oceania chair of the IUCN/WI Freshwater Fish Specialist Group, Aaron Jenkins and partners, including the Fiji Department of Fisheries. It shows the degree of invasiveness and the deleterious effects of invasive fishes in the Pacific island nation of Fiji.

Invasion damage

The study shows firstly that a large portion of the country’s catchments (and all major catchments) are already invaded, primarily with Tilapia (Oreochromis spp.) and Mosquitofish (Gambusia affinis) – (Figure 1). For the 89 catchments surveyed – 85.4% were invaded and only 14.6% were invasive free. Secondly, it is shows that native and endemic (exclusively native) fish fauna are being locally extinct or extirpated, which in the context of small islands can be well on the road to extinction.

Figure 1. click to enlarge.

Mid-river sample sites in 20 catchments showed that total fish species dropped by 11 species when Tilapia is present (Figure 2). No less than 5 out of 9 of the Fijian endemic species were missing from these assemblages. Several of the large native edible fishes (Vo in Fijian) were also missing from these assemblages, showing that invasive fishes are also reducing traditional sources of food security.

Migratory fish also diminishing

Migratory fishes are usually amphidromous (a particular type of migration between sea and freshwater) in Pacific Islands. The study also shows that mid-reach sites with invasive Tilapia had 7 fewer amphidromous species (Figure 2). This shows that that the fundamental processes of migration are also being disrupted by the presence of invasive fishes, reducing the faunal connectivity between oceans and freshwater. The mechanism of depletion of native biodiversity is likely through a combination of predation (Pacific islands naturally have few mid-water predators) and reduction of water quality through suspension of sediment during nesting and foraging.

Source of invasive species

As many wild invasions come from poorly constructed and managed aquaculture facilities adjacent to rivers, it is critical that these construction and husbandry standards are well regulated at international, national and community levels. It is also necessary to increase the aquaculture of native species that do not pose such dire threat to natural systems.

Figure 2. Click to enlarge.

Therefore, Wetlands International calls for aquaculture with exclusively endemic or native species in high conservation freshwater areas.